Short answer

HubSpot's KB defines the deal property Original Traffic Source as the original source of the associated contact with the earliest activity, with matching drill-down properties. Check it before building a copy workflow, as one client task originally planned. If you need a different contact's value or another property, the Property sync field type or a workflow's Edit record action can copy it. For the workflow, the association must exist before the action runs.

1. The property already exists

Deal properties in my demo portal, searched for original traffic.

Deal properties list filtered to original traffic showing the Original Traffic Source property — Deal properties (1), the search (2), and Original Traffic Source (3).

2. When you need your own copy

The create property panel has a Property sync field type that asks which associated record type to read from. I opened it without saving.

Create property panel with Field type Property sync and Choose the associated record type — Property sync (1) and the associated record type (2).

3. Or copy with a workflow

HubSpot's KB says to enroll when the source property is known, and that the association must exist before the Edit record action.
A contact can have several deals. Decide whether all of them, or only open ones, should get the value.
For which contact counts, see traffic sources explained and deal attribution requirements.

Getting a contact's traffic source onto the deal: check the default property before building a workflow

Short answer

1. The property already exists

2. When you need your own copy

3. Or copy with a workflow

Sources

New field notes by email

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